1. Field of the Invention
The present invention relates to electronic dimming ballasts operable to be coupled to a communication link, specifically, a communication circuit for an electronic dimming ballast that provides high-voltage miswire protection and improved rise and fall times of a transmitted digital signal.
2. Description of the Related Art
Typical load control systems are operable to control the amount of power delivered to an electrical load, such as a lighting load or a motor load, from an alternating-current (AC) power source. Lighting control systems for fluorescent lamps comprise a plurality of electronic dimming ballasts that are operable to communicate via a digital communication link. The ballasts may communicate, for example, using the industry-standard Digital Addressable Lighting Interface (DALI) communication protocol. The DALI protocol allows each ballast in the lighting control system to be assigned a unique digital address, to be programmed with configuration information (e.g., preset lighting intensities), and to control the fluorescent lamp in response to commands transmitted across the communication link.
A standard DALI lighting control system includes a link power supply that generates a direct-current (DC) link voltage VLINK, which provides power for the DALI communication link. The DALI communication link comprises two conductors (i.e., two wires) and is coupled to each of the ballasts, such that each ballast receives the DC link voltage VLINK of the link power supply. The ballasts are also coupled to the AC power source to receive line voltage (e.g., 120 or 277 VAC) for powering the fluorescent lamps. To simplify installation, the two conductors of the DALI communication link are often installed in the same channel or conduit as the high-voltage AC wiring (i.e., line voltage). Thus, the conductors of the DALI communication link are often classified as “high-voltage” conductors.
Each DALI ballast includes a microprocessor for handling the communications with the other DALI ballasts and controlling the operation of the circuitry that controls the intensity of the connected lamp. A communication circuit in each of the DALI ballasts couples the microprocessor to the DALI communication link. The communication circuit preferably comprises at least two optocouplers for providing the received digital messages to the microprocessor and for providing the digital messages to be transmitted on the DALI communication link. The optocouplers provide isolation between the conductors of the DALI communication link and the microprocessor.
According to the DALI protocol, the DALI ballasts encode the digital messages that are transmitted over the communication link using Manchester encoding. With Manchester encoding, the bits of the digital messages (i.e., either a logic zero value or a logic one value) are encoded in the transitions (i.e., the edges) of the signal on the communication link. When no messages are being transmitted on the communication link, the link floats high in an idle state. To transmit a logic one value, the communication circuit of each DALI ballast is operable to “short” the communication link (i.e., electrically connect the two conductors of the link) to cause the communication link to change from the idle state (i.e., 18 VDC) to a shorted state (i.e., a “high-to-low” transition). Conversely, to transmit a logic zero value, the communication circuit is operable to cause the communication link to transition from the shorted state to the idle state (i.e., a “low-to-high” transition).
An example of a digital electronic dimming ballast operable to be coupled to a communication link and a plurality of other input sources is described in greater detail in co-pending commonly-assigned U.S. patent application Ser. No. 10/824,248, filed Apr. 14, 2004, entitled MULTIPLE-INPUT ELECTRONIC BALLAST WITH PROCESSOR, and U.S. patent application Ser. No. 11/011,933, filed Dec. 14, 2004, entitled DISTRIBUTED INTELLIGENCE BALLAST SYSTEM AND EXTENDED LIGHTING CONTROL PROTOCOL. The entire disclosures of both applications are hereby incorporated by reference.
The DALI protocol is standardized by the in accordance with a technical standard published by the International Electrotechnical Commission (IEC), specifically, the IEC standard 60929A, which defines many required operating characteristics of the communication circuit of a DALI ballast. The technical standard imposes limitations on the current draw of the DALI ballast. For example, when the communication link is idle (i.e., 18 VDC), the communication circuit must not draw more than 2 mA. When the communication circuit is transmitting (i.e., shorting the link), the DALI ballast must draw at least 250 mA and must provide no more than 4 V between the conductors of the communication link. The IEC standard also defines the rise and fall times of the edges of the data signal to be between 10 μs and 100 μs.
The use of optocouplers to transmit digital messages on the DALI communication link often causes the digital messages transmitted by the DALI ballasts to be susceptible to long rise and fall times. While some characteristics, such as the current transfer ratio (CTR), of the optocoupler are guaranteed by an optocoupler manufacturer, the rise and fall times are typically specified only under specific operating conditions. Therefore, the rise and fall times of the optocoupler cannot be guaranteed, for example, when the optocouplers are used to drive the DALI communication link, unless the same operating conditions exist. To decrease the length of the rise and fall times of the data signal in order to meet the IEC standard, it is often necessary to drive the optocouplers with larger currents. However, these drive currents cannot exceed the maximum idle current limit (i.e., 2 mA) when the link is idle.
Since the two conductors of the DALI communication link are often run along side the high-voltage wiring for the ballasts, it is possible that the two conductors of the DALI communication link may be miswired to the high-voltage wiring. The communication circuits of many prior art DALI ballasts have not been protected against high-voltage (i.e., line voltage) miswires. Some prior art ballast have simply included all high-voltage rated components in the communication circuits. However, if such a communication circuit shorts the communication link during a high-voltage miswire, the communication circuit is susceptible to damage due to high currents that are generated.
Thus, there is a need for a DALI communication circuit that is able to reliably transmit digital messages having rise and fall times that are within the range defined by the IEC standard, while also meeting the current draw limitations of the IEC standard. Further, there is a need for a DALI communication circuit that is capable of being miswired to high-voltage mains voltages, such as 120 or 277 VAC, without having components damaged under any circumstances.